1. Field of the Invention
The present invention relates to a control device for an internal combustion engine, and more particularly to a control device for an internal combustion engine, which utilizes an IMEP or an IMEP variation amount calculated from an ionic volume, which is generated by combustion occurring in an internal combustion engine, to thereby control various internal combustion engine control parameters.
2. Description of the Related Art
In recent years, in view of strict environmental restrictions, there is increased need to detect limits in engine combustion, in particular a lean combustion limit and an exhaust gas recirculation (hereinafter referred to as EGR) limit, in order to improve fuel mileage.
The combustion limit is a point where the internal combustion engine (hereinafter sometime referred to as engine) is operated most effectively and in a stable fashion. Namely, what is important is how much fuel can be reduced while obtaining the same output, and achieving stable combustion to maintain product quality.
A cylinder pressure sensor is generally used as an indicator to measure these factors. An indicated mean effective pressure (hereinafter referred to as IMEP for short), obtained from the pressure inside the cylinder serves as the indicator.
However, there are problems in terms of the cost and durability of the cylinder pressure sensor, thus rendering it impractical.
A technique has been proposed, as one application of an ion current detector, which detects the combustion limit. This device uses as a probe a spark plug to detect a current of the ions generated during the combustion. (E.g., JP 11-324881 A)
However, the combustion limit detection method proposed in JP 11-324881 A only detects a limit point up to which combustion is possible. The method does not take into consideration the limit point at which the most efficient output can be obtained, nor detection of the combustion stability needed to secure a product quality of the engine. Thus, these kinds of detection were difficult to achieve with the technique proposed in JP 11-324881 A.
In order to control work efficiency of the engine, it is necessary to detect the IMEP, which is the actual combustion work. Furthermore, in order to discuss the stability of the engine, which contributes to secure the product quality of the engine, it is important to deal with the amount of variation occurring in the combustion work, to thereby achieve detection before variation occurs in the engine itself.